The total bandwidth in an Orthogonal Frequency Division Multiplexing (OFDM) system is divided into narrowband frequency units called subcarriers. The number of subcarriers is equal to the Fast Fourier Transform/inverse Fast Fourier Transform (FFT/IFFT) size N used in the system.
Currently-implemented OFDM networks use either a frequency division duplexing (FDD) or a time division duplexing (TDD) scheme. In FDD-based communication systems, the uplink uses a different frequency band from the downlink. Typically, these systems make use of symmetric spectrum allocation, i.e., the bandwidth allocated to the downlink is the same as in the uplink. Disadvantages of this approach include the use of a fixed spectrum band that is inflexible to reconfiguration of the uplink and downlink bandwidth to support different data rate and capacity requirements for different classes of service. In TDD-based communication systems, the uplink and the downlink share the same frequency band, but the uplink and downlink transmissions occur at different times. Thus, these systems provide flexible use of the spectrum between uplink and downlink. The disadvantages with this approach include delays in transmissions due to having to wait for the allotted transmission time and lower link budgets due to mobile stations not being able to transmit continuously.
The use of a sub-carrier division duplexing (SDD) scheme overcomes the disadvantages of FDD and TDD schemes by allowing the allocation of sub-carriers to be changed dynamically at any time based on network conditions. Thus, for example, if additional sub-carriers are needed for the downlink while the uplink is not being fully used, a base station may allocate more sub-carriers as downlink sub-carriers and less sub-carriers as uplink sub-carriers.
Edge guard sub-carriers are operable to provide a guard to protect against interference between communication using the sub-carriers assigned to the base station and frequency bands that may be assigned to other systems. Similarly, transition guard sub-carriers are operable to provide a guard to protect against interference between communication on the downlink sub-carriers and communication on the uplink sub-carriers.
Further description of dynamically allocating sub-carriers can be found in U.S. Published Patent Application No. 2006/0209755 A1, which is hereby incorporated by reference into the present application as if fully set forth herein.
Like OFDMA, a single-carrier FDMA (SC-FDMA) scheme also provides orthogonal access to multiple users simultaneously accessing the system. Another attractive feature of SC-FDMA in comparison to OFDMA is a low peak-to-average power ratio (PAPR) due to its single carrier transmission property. In an SC-FDMA scheme referred to as Interleaved Frequency Division Multiple Access (IFDMA), a data sequence is first repeated for a predetermined number of times. The repeated data sequence is then multiplied with a user-specific phase vector.
Another way of looking at this approach is FFT preceding the data sequence and then mapping the FFT-precoded data sequence to uniformly spaced subcarriers at the input of the IFFT. The uniform spacing is determined by the repetition factor Q. The multiplication of the repeated data sequence with a user-specific phase vector can be seen as a frequency shift in order to map transmissions from multiple users on non-overlapping orthogonal subcarriers. Although having each data modulation symbol spread out on all the subcarriers used by the user can provide frequency-diversity benefit in a frequency selective channel, there may be some impact on performance as well due to the loss of orthogonality or noise enhancement when data symbols experience frequency selective fading.
The mapping of FFT-precoded data sequence to contiguous subcarriers results in a localized transmission in the frequency domain. Similar to distributed mapping or DFDMA, localized mapping also results in a low PAPR signal. The distributed and localized mapping of FFT pre-coded data sequence to OFDM subcarriers is sometimes collectively referred to as Discrete Fourier Transform-Spread (OFDM DFT-Spread) OFDM.
Further description of FFT-precoding can be found in U.S. Published Patent Application No. 2006/0227888 A1, which is hereby incorporated by reference into the present application as if fully set forth herein.
However, in general, the number of subcarriers used for data transmission is still less than N because some subcarriers are used as transition guard sub-carriers, and typically no information is transmitted on transition guard sub-carriers.
Therefore, there is a need in the art for a system and method for allocating sub-carriers for communication in an OFDM network that allows the transition guard sub-carriers to be used for data transmission. In particular, there is a need for a subcarrier division duplexing scheme that allows the transition guard sub-carriers to be used for data transmission by neighbor cells.